The multiple causes make it harder to do something about what’s called colony collapse disorder, experts say. The disorder has caused as much as one-third of the nation’s bees to just disappear each winter since 2006.

Bees, especially honeybees, are needed to pollinate crops.

The federal report, issued Thursday by the Agriculture Department and the Environmental Protection Agency, said the biggest culprit is the parasitic mite varroa destructor, calling it “the single most detrimental pest of honeybees.”